High melt flow polymer of improved durability for pipe applications

ABSTRACT

The present invention relates to a polymer composition, comprising (i) a polypropylene homopolymer, (ii) a polypropylene random copolymer, prepared by copolymerization of propylene with an olefin comonomer and having an amount of olefin comonomer units of 0.2 to 5 wt %, and (iii) an elastomeric copolymer of propylene and at least one olefin comonomer, the polymer composition having a tensile modulus, determined according to ISO 527-2/1 B at 1 mm/min. and 230 C, of at least 1200 MPa.

The present invention relates to a polymer composition having improveddurability and stiffness while still keeping processability on a highlevel. Furthermore, it relates to a process for preparing such a polymercomposition and to pipes made therefrom.

Pipes made of polymeric material are frequently used for variouspurposes such as fluid transport, i.e. transport of liquids and gases.The fluid may be pressurized, e.g. when transporting natural gas or tapwater, or non-pressurized, e.g. when transporting sewage (waste-water),drainage, for storm water applications or indoor sewage (soil and wastedischarge). Moreover, the transported fluid may have varyingtemperature, usually within the range of 0° C. to 50° C. Pressureless(non-pressure) pipes may also be used for cable and pipe protection andfor culverts (e.g. road and rail).

Polypropylene-based polymers have many characteristics which make themsuitable for applications like pipes, fittings, moulded articles, foamsetc. Polypropylene as pipe material is mainly used in non-pressureapplications (pipes and fittings) and profiles. Polypropylene is alsoused for pressure pipes, mainly for hot water and industrial pipes. Thegood properties at high temperature of polypropylene compared to otherpolyolefins are often utilized for pipe applications. All three maintypes of propylene polymers, i.e. homopolymers, random copolymers andblock copolymers (i.e. rubber-like elastomeric copolymers) are used.

In general, polypropylene-based materials to be chosen for pipeapplications should result in products of high stiffness as well as highdurability while still maintaining good processability. However, theseproperties are interrelated to each other and very often behave in aconflicting manner, i.e. improving a specific property can only beaccomplished on the expense of another property.

Stiffness can be improved by increasing the amount of homopolymer withinthe composition. As a consequence, the material becomes more brittle,thereby resulting in poor impact properties. Furthermore, highbrittleness is usually accompanied by lower resistance to slow crackgrowth, thereby having a detrimental effect on durability.

Pipes are normally produced by extrusion or, to a smaller extent, byinjection moulding. Thus, to improve processability the polymeric meltshould be of low viscosity. However, by using such low viscositymaterial, the resulting durability is usually not sufficient for meetingthe requirements as defined in international standards like EN 1852,prEN13476. Normally, melt flow rate values of polymers for non-pressurepipe applications are kept at a low level to fulfil standardrequirements on durability.

Furthermore, non-pressure pipes need to have sufficient ring stiffnessto withstand the soil pressure. High stiffness is indicated by highvalues of tensile modulus. However, as explained above, an increase intensile modulus and, consequently, in stiffness of the pipe might have adetrimental effect on impact properties and/or slow crack growthresistance.

In EP 0894103, a polymer composition is provided comprising apolypropylene homopolymer and a polypropylene random copolymer.Optionally, a rubbery copolymer can be added to result in a so-calledimpact modified polymer. For these impact modified polymers, melt flowrate MFR_(2.16 kg/230° C.) is kept at a low level. Furthermore, atensile modulus is chosen which does not exceed 1100 MPa.

Considering the problems discussed above, it is an object of the presentinvention to provide a polypropylene-based composition from which pipesof high stiffness as well as high durability can be prepared while stillmaintaining good processability of the polymeric melt. A further objectis to provide a process for the preparation of such a composition and toprovide a pipe of high stiffness and durability and a low amount ofdefects due to the improved processability of the polymer.

These objects are solved by providing a polymer composition, comprising

-   (i) a polypropylene homopolymer,-   (ii) a polypropylene random copolymer, prepared by copolymerization    of propylene with an olefin comonomer and having an amount of olefin    comonomer units of 0.2 to 5.0 wt %, and-   (iii) an elastomeric copolymer of propylene and at least one olefin    comonomer,    the polymer composition having a tensile modulus, determined    according to ISO 527-2/1 B at 1 mm/min and 23° C., of at least 1200    MPa.

Within the context of the present invention, a polypropylene homopolymeris defined to be a polymer preferably consisting of more than 99.8 wt %,more preferably more than 99.9 wt %, even more preferably more than99.99 wt % of propylene units.

If units other than propylene units are present, these originatepreferably from an olefin such as ethylene.

The polypropylene homopolymer can be unimodal or multimodal.

The expression “multimodal” used herein refers to the modality of thepolymer, i.e. the form of its molecular weight distribution curve, whichis the graph of the molecular weight fraction as a function of itsmolecular weight. As will be explained below, the polymer components ofthe present invention can be produced in a sequential step process,using reactors in serial configuration and operating at differentreaction conditions. As a consequence, each fraction prepared in aspecific reactor will have its own molecular weight distribution. Whenthe molecular weight distribution curves from these fractions aresuperimposed to obtain the molecular weight distribution curve of thefinal polymer, that curve may show two or more maxima or at least bedistinctly broadened when compared with curves for the individualfractions. Such a polymer, produced in two or more serial steps, iscalled bimodal or multimodal, depending on the number of steps.

In a preferred embodiment, the polypropylene homopolymer fraction has amelt flow rate MFR_(2.16 kg/230° C.) of less than 20 g/10 min. Ingeneral, melt flow rate is related to flowability of the polymeric meltat a specific temperature when subjected to a specific load. High meltflow rate values indicate a polymeric melt of low viscosity, and viceversa.

As will be explained below in greater detail, the polypropylenehomopolymer can be prepared in a first step, i.e. before preparing thepolypropylene random copolymer and the elastomeric copolymer, or at alater stage. If prepared in a later stage, the homopolymer can bereactor-blended with a component already prepared before, therebyresulting in a polymeric mixture from which only a total melt flow ratecan be determined but not the melt flow rate of each component withinthe blend. However, even if obtained as a reactor blend, theMFR_(2.16 kg/230° C.) of the polypropylene homopolymer refers to a purehomopolymer fraction. In other words, it is the melt flow rate value ofthe polypropylene homopolymer that would have been obtained ifpolymerized without the presence of further components.

In other preferred embodiments, the polypropylene homopolymer fractionhas a melt flow rate MFR_(2.16 kg/230° C.) of less than 10 g/10 min.,less than 5 g/10 min., or even less than 2 g/10 min.

A further essential feature of the polymer composition of the presentinvention is the presence of a polypropylene random copolymer, preparedby copolymerization of propylene with an olefin comonomer and having anamount of olefin comonomer units of 0.2 to 5.0 wt %, based on the weightof the polypropylene random copolymer.

As already indicated above, a random copolymer is one of the three maintypes of polypropylene. In general, a polypropylene random copolymer isobtained when propylene is polymerized with at least one comonomer so asto result in a random or statistical distribution of the comonomerwithin the polymer chain. The amount and type of comonomer has asignificant influence on a number of properties like crystallizationbehaviour, stiffness, melting point or flowability of the polymer melt.To solve the objects of the present invention, in particular to providean improved balance between stiffness, durability and melt flowbehaviour, it is necessary to keep the amount of comonomer units of thepolypropylene random copolymer within the range given above.

Preferably, the amount of comonomer units within the polypropylenerandom copolymer is from 0.5 wt % to 4.0 wt %. In other preferredembodiments, the amount is from 0.5 wt % to 3.0 wt %, or from 0.8 wt %to 3.0 wt %.

Preferably, the polypropylene random copolymer has a weight averagemolecular weight which is higher than the weight average molecularweight of the polypropylene homopolymer. Since molecular weight and meltflow rate are in a reciprocal relation to each other, it is alsopreferred that the polypropylene random copolymer has aMFR_(2.16 kg/230° C.) which is lower than the MFR_(2.16 kg/230° C.) ofthe polypropylene homopolymer.

The olefin comonomer is preferably selected from ethylene, C₄ to C₁₀alpha-olefins such as 1-butene, 1-hexene or 1-octene, or mixturesthereof. Ethylene is the preferred comonomer.

When ethylene is used as the olefin comonomer, its amount within thepolypropylene random copolymer is preferably within the range of 0.2 to5.0 wt %. In other preferred embodiments, its amount is within the rangeof 0.5 wt % to 4.0 wt %, 0.5 wt % to 3.0 wt %, or 0.8 wt % to 3.0 wt %.

Even at a very low level of comonomer within the polypropylene randomcopolymer, the polymer composition of the present invention is stillsuccessful in simultaneously improving durability and processability ata high stiffness level.

When based on the combined weight of the polypropylene homopolymer andthe polypropylene random copolymer, the amount of ethylene comonomerunits within the random copolymer is preferably from 0.1 wt % to 3.0 wt%. In other preferred embodiments, the amount is preferably from 0.1 wt% to 2.0 wt %, or from 0.5 wt % to 1.5 wt %.

In a preferred embodiment, the polypropylene homopolymer and thepolypropylene random copolymer are prepared in the same reactor or intwo or more reactors connected to each other in serial configuration, aswill be explained below in greater detail. As a consequence, bothcomponents are reactor-blended during polymerization and result in apolypropylene-based matrix.

Preferably, the polypropylene-based matrix has a melt flow rateMFR_(2.16 kg/230° C.) of 0.1 to 10.0 g/10 min. In other preferredembodiments, the matrix has a MFR_(2.16 kg/230° C.) of 0.2 to 5.0 g/10min, 0.2 to 3.5 g/10 min., 0.2 to 2.0 g/10 min, 0.2 to 1.5 g/10 min, or0.2 to 1.0 g/10 min.

The melting point of the matrix is preferably above 157° C., above 158°C., or above 159° C.

As already indicated above, the matrix preferably has an amount ofethylene comonomer units of 0.1 wt % to 3.0 wt %, based on the weight ofthe matrix. In other preferred embodiments, the amount is preferablyfrom 0.1 wt % to 2.0 wt %, or from 0.5 wt % to 1.5 wt %.

A further essential feature of the polymer composition of the presentinvention is the presence of an elastomeric copolymer of propylene andat least one olefin comonomer. The presence of such an elastomericpropylene copolymer improves impact performance of the final polymer.The conditions for the copolymerization are within the limits ofconventional conditions for ethylene-propylene rubber (EPM) production.Typical conditions are disclosed e.g. in Encyclopedia of Polymer Scienceand Engineering, second edition, vol. 6, p. 545-558. An elastomericproduct is obtained when the comonomer content of the polymer is withina certain range.

Suitable olefin comonomers to be copolymerized with propylene can beselected from ethylene, C₄ to C₁₀ alpha-olefins such as 1-butene,1-hexene or 1-octene, or mixtures thereof. Preferably, ethylene is used.

Preferably, the elastomeric copolymer contains olefin comonomer units inan amount of 10 to 70 wt %, more preferably 20 to 50 wt %, based on theweight of the elastomeric copolymer.

When ethylene is used as an olefin comonomer, the elastomeric copolymerpreferably contains ethylene units in an amount of 10 to 70 wt %, basedon the weight of the elastomeric copolymer. In other preferredembodiments, the amount of ethylene units within the elastomericcopolymer is from 20 wt % to 50 wt %, from 25 wt % to 40 wt %, or from30 wt % to 40 wt %.

Preferably, the elastomeric copolymer has an intrinsic viscosity of 2 to6 dl/g, measured in tetraline. More preferably, intrinsic viscosity iswithin the range of 3 to 5 dl/g. Intrinsic viscosity is related to themolecular weight, i.e. intrinsic viscosity increases with increasingmolecular weight. It is measured as follows: The relative viscosity of apolymer sample dissolved in tetraline (1,2,3,4-tetrahydronaphthalene) ismeasured by using a Ubbelohde viscometer when the concentration of thepolymer sample is 0.1 g/100 ml. The intrinsic viscosity is the valuecalculated with the Huggins equation.

As indicated above, the presence of an elastomeric ethylene-propylenecopolymer improves impact properties as determined e.g. by measurementof Charpy notched impact strength. However, if the amount of elastomericcopolymer within the final polymer composition is too high, this mighthave a detrimental effect on other properties like stiffness orprocessability.

Preferably, the polymer composition of the present invention comprisesan amount of elastomeric copolymer within the range of 5 wt % to 15 wt%, more preferably 7 wt % to 15 wt %, based on the weight of the polymercomposition.

According to a further essential feature, the polymer composition of thepresent invention needs to have a tensile modulus of at least 1200 MPa,thereby resulting in a material of high stiffness. Tensile modulus isdetermined according to ISO 527-2/1 B at 1 mm/min. and 23° C. A tensilemodulus of the final polymer composition of at least 1200 MPa incombination with the specific components (i), (ii) and (iii) enables tohave high stiffness while simultaneously improving durability andprocessability.

Preferably, the polymer composition of the present invention has atensile modulus of at least 1300 MPa, at least 1350 MPa, at least 1400MPa, at least 1450 MPa, at least 1500 MPa, at least 1550 MPa or at least1600 MPa.

A property related to durability of a polymeric material is theresistance to slow crack growth. This property can be tested accordingto ISO 1167, wherein the resistance of a pipe made of the polymercomposition to a circumferential (hoop) stress of 4.2 MPa at a constanttemperature of 80° C. is determined. The time to failure of the pipe isrecorded.

As discussed above, melt flow rate and durability are contradictingproperties. To have an improved balance between these properties, thepolymer composition of the present invention preferably has a melt flowrate MFR_(2.16 kg/230° C.) and a resistance to slow crack growth whichsatisfy the following relationship:

t≧300−200*MFR_(2.16 kg/230° C.) for 0.1 g/10min≦MFR_(2.16 kg/230° C.)<1.0 g/10 min,

t≧105−5*MFR_(2.16 kg/230° C.) for 1.0 g/10min≦MFR_(2.16 kg/230° C.)<10.0 g/10 min,

t≧55 for MFR_(2.16 kg/230° C.)≧10.0 g/10 min,

wherein t in hours is the time of failure in the slow crack growth testperformed at 80° C. and 4.2 MPa according to ISO 1167.

In a further preferred embodiment, the polymer composition of thepresent invention has a melt flow rate MFR_(2.16 kg/230° C.) and aresistance to slow crack growth which satisfy the followingrelationship:

t≧500−350*MFR_(2.16 kg/230° C.) for 0.1 g/10min≦MFR_(2.16 kg/230° C.)<1.0 g/10 min,

t≧155−5*MFR_(2.16 kg/230° C.) for 1.0 g/10min≦MFR_(2.16 kg/230° C.)<10.0 g/10 min,

t≧105 for MFR_(2.16 kg/230° C.)≧10.0 g/10 min,

wherein t has the same meaning as indicated above.

Even more preferably, the polymer composition of the present inventionhas a melt flow rate MFR_(2.16 kg/230° C.) and a resistance to slowcrack growth which satisfy the following relationship:

t≧700−500*MFR_(2.16 kg/230° C.) for 0.1 g/10min≦MFR_(2.16 kg/230° C.)<1.0 g/10 min,

t≧205−5*MFR_(2.16 kg/230° C.) for 1.0 g/10min≦MFR_(2.16 kg/230° C.)<10.0 g/10 min,

t≧155 for MFR_(2.16 kg/230° C.)≧10.0 g/10 min,

wherein t has the same meaning as indicated above.

To improve flowability, the polymer composition preferably has a meltflow rate MFR_(2.16 kg/230° C.) of at least 0.2 g/10 min.

In a preferred embodiment, the melt flow rate MFR_(2.16 kg/230° C.) ofthe polymer composition is within the following range:

0.2 g/10 min≦MFR_(2.16 kg/230° C.)<10.0 g/10 min,

to have a better compromise between flowability of the polymeric meltand durability of the final polymer.

In other preferred embodiments, the melt flow rate MFR_(2.16 kg/230° C.)of the polymer composition is 0.2 to 8.0 g/10 min, 0.2 to 7.0 g/10 min.0.2 to 6.0 g/10 min, 0.2 to 5.0 g/10 min, or 0.2 to 3.0 g/10 min.

The polymer composition of the present invention preferably has a Charpyimpact strength (NIS) at −20° C. of at least 2.0 kJ/m², more preferablyat least 3.0 kJ/m², even more preferably at least 4.0 kJ/m², and mostpreferably at least 5.0 kJ/m², measured according to ISO 179/1eA.

Furthermore, to optimize the balance between stiffness and impactbehaviour at low temperature, the tensile modulus of the polymercomposition is preferably chosen to be at least 1375 MPa whereas theCharpy impact strength at −20° C. is at least 3.0 kJ/m². In otherpreferred embodiments, the following combinations of tensile modulus andCharpy impact strength are chosen: at least 1400 MPa and at least 3.5kJ/m², at least 1400 MPa and at least 4.0 kJ/m², or at least 1450 MPaand at least 3.5 kJ/m².

Preferably, the polymer composition has a polydispersion index PI of 2.5to 6.0. In other preferred embodiments, PI of the matrix is within therange of 3.0 to 5.0 or 3.0 to 4.5. The polydispersion index PI iscalculated according to the following equation:

PI=10⁵ Pa/G _(C)

wherein G_(C) in Pa is the cross over modulus at which G′=G″=G_(C), G′and G″ indicating the storage modulus and the loss modulus,respectively.

The rheology measurements have been done according to ISO 6421-10.Measurements were made at 220° C. and 200° C. Further details about G′and G″ and the measuring method are provided below in the examples.

In addition to the components discussed above, the polymer compositionmay comprise conventional adjuvants, such as additives, fillers andreinforcing agents.

As additives, the following can be mentioned: nucleating agents, processand heat stabilizers, pigments and other colouring agents includingcarbon black. Depending on the type of additive, these may be added inan amount of 0.01 to 5 wt %, based on the weight of the polymercomposition.

In a preferred embodiment, the polymer composition includes 0.05 to 3 wt%, based on the weight of the polymer composition, of one or morealpha-nucleating agents such as talc, polymerized vinyl compounds suchas polyvinyl cyclohexane, dibenzylidene sorbitol, sodium benzoate, anddi(alkylbenzylidene)sorbitol. Except for talc, the alpha-nucleatingagent is usually added in small amounts of 0.0001 to 1.0 wt %, morepreferably 0.001 to 0.7 wt %. Since talc can act both as a nucleatingagent and as a filler, it can be added in higher amounts. When added asa nucleating agent, talc is preferably added in an amount of 0.05 to 3.0wt %, more preferably 0.1 to 2.0 wt %, based on the weight of thepolymer composition. Further details about these nucleating agents canbe found e.g. in WO 99/24479 and WO 99/24501.

According to the present invention, there is also provided a process forpreparing the polymer composition discussed above. The process of thepresent invention comprises the following steps:

(i) preparing a polypropylene homopolymer,(ii) copolymerization of propylene with an olefin comonomer to result ina polypropylene random copolymer, and(iii) copolymerization of propylene with an olefin comonomer to resultin an elastomeric copolymer,wherein these steps can be carried out in any sequence.

However, it is preferred to either have the sequence (i)→(ii)→(iii) orthe sequence (ii)→(i)→(iii).

Preferably, at least the process steps (i) and (ii), in any sequence,are carried out in at least one loop reactor and/or at least one gasphase reactor. According to another preferred embodiment, all processsteps (i) to (iii) are carried out in at least one loop reactor and/orat least one gas phase reactor.

According to a preferred embodiment, the first reaction step is carriedout in a loop reactor, this step optionally also comprising at least onegas phase reactor to which the product of the loop reactor istransferred to continue polymerization. Preferably, any reaction mediumused and any non-reacted reagents are at least partly removed beforetransfer from the loop reactor to the gas phase reactor is performed.

For the present invention, conventional loop and gas phase reactorswhich are commonly known in the relevant technical field can be used.

If the polypropylene homopolymer is prepared first, reaction conditionsare chosen so as to preferably have a MFR_(2.16 kg/230° C.) of less than20 g/10 min for the homopolymer. By using a loop reactor and at leastone gas phase reactor in serial configuration and working at differentconditions, a multimodal (e.g. bimodal) polypropylene homopolymer can beobtained. However, within the context of the present invention, thepolypropylene homopolymer can also be unimodal.

As an alternative, the polypropylene random copolymer is prepared first.Again, polymerization can be effected by using a loop reactor only or aloop reactor in serial configuration with at least one gas phasereactor, the latter configuration resulting in a multimodal (e.g.bimodal) polypropylene random copolymer.

The amount and feed rate of olefin comonomer fed into the reactor forcopolymerization with propylene are such that the random copolymer hasan amount of olefin comonomer units of 0.2 to 5.0 wt %, based on theweight of the random copolymer. Preferably, ethylene is used as theolefin comonomer.

In a preferred embodiment, the polypropylene homopolymer is preparedfirst in a loop reactor. Preferably, the homopolymer has aMFR_(2.16 kg/230° C.) of less than 20 g/10 min, more preferably lessthan 10 g/10 min. and even more preferably less than 5 g/10 min or lessthan 2 g/10 min. Subsequently, the polypropylene homopolymer istransferred to a first gas phase reactor wherein copolymerization ofpropylene with an olefin comonomer, preferably ethylene, to thepolypropylene random copolymer is effected, thereby resulting in areactor-blended polymeric mixture, i.e. a polypropylene-based matrix,having an amount of comonomer units of 0.1 wt % to 3.0 wt %, based onthe weight of the matrix. In other preferred embodiments, the amount ispreferably from 0.1 wt % to 2.0 wt %, or from 0.5 wt % to 1.5 wt %.

Preferably, a loop reactor for preparing a polypropylene homopolymer orrandom copolymer is operated at a temperature of 60° C. to 95° C. and apressure of 4000 kPa to 8000 kPa. In a preferred embodiment, at leastone loop reactor is operated under supercritical conditions. As anexample, supercritical conditions can include a temperature of at least92° C. and a pressure of at least 4600 kPa.

Preferably, a gas phase reactor for preparing a polypropylenehomopolymer or random copolymer is operated at a temperature of 60° C.to 100° C. and a pressure of 1000 kPa to 4000 kPa.

To further improve the balance between stiffness, durability andprocessability of the polymer composition, a specific split between afirst process step producing a first component and a second process stepproducing a second component can be chosen. The split indicates theweight ratio between different polymeric components prepared indifferent reaction steps. Preferably, the split between process step (i)and process step (ii), irrespective of their sequence, is from 80:20 to20:80, more preferably from 70:30 to 30:70 and even more preferably from40:60 to 60:40

As a catalyst for the preparation of the polypropylene homopolymerand/or the polypropylene random copolymer, any stereo-specific catalystfor propylene polymerization can be used, which is capable of catalyzingpolymerization and copolymerization of propylene and comonomers at apressure of 500-10000 kPa, in particular 2500-8000 kPa, and at atemperature of 40-110° C., in particular 60-110° C.

Preferably, the catalyst comprises a high-yield Ziegler-Natta typecatalyst which can be used at high polymerization temperatures of 80° C.or more.

Generally, the Ziegler-Natta catalyst used in the present inventioncomprises a catalyst component, a cocatalyst component, an externaldonor, the catalyst component of the catalyst system primarilycontaining magnesium, titanium, halogen and an internal donor. Electrondonors control the stereospecific properties and/or improve the activityof the catalyst system. A number of electron donors including ethers,esters, polysilanes, polysiloxanes, and alkoxysilanes are known in theart.

The catalyst preferably contains a transition metal compound as aprocatalyst component. The transition metal compound is selected fromthe group consisting of titanium compounds having an oxidation degree of3 or 4, vanadium compounds, zirconium compounds, cobalt compounds,nickel compounds, tungsten compounds and rare earth metal compounds,titanium trichloride and titanium tetrachloride being particularlypreferred.

It is preferred to use catalysts which can withstand the hightemperatures prevailing in the loop reactor. The conventionalZiegler-Natta catalysts for isotactic polymerization of propylenegenerally have an operating temperature limit of around 80° C., abovewhich they either become deactivated or lose their stereo-selectivity.This low polymerization temperature may put a practical limit on theheat removal efficiency of the loop reactor.

One preferred catalyst to be used according to the invention isdisclosed in EP 591 224 which discloses a method for preparing aprocatalyst composition from magnesium dichloride, a titanium compound,a lower alcohol and an ester of phthalic acid containing at least fivecarbon atoms. According to EP 591 224, a trans-esterification reactionis carried out at an elevated temperature between the lower alcohol andthe phthalic acid ester, whereby the ester groups from the lower alcoholand the phthalic ester change places.

Magnesium dichloride can be used as such or it can be combined withsilica, e.g. by absorbing the silica with a solution or slurrycontaining magnesium dichloride. The lower alcohol used may preferablybe methanol or ethanol, particularly ethanol.

The titanium compound used in the preparation of the procatalyst ispreferably an organic or inorganic titanium compound, which is at theoxidation state of 3 or 4. Also other transition metal compounds, suchas vanadium, zirconium, chromium, molybdenum and tungsten compounds canbe mixed with the titanium compound. The titanium compound usually is ahalide or oxyhalide, an organic metal halide, or a purely metal organiccompound in which only organic ligands have been attached to thetransition metal. Particularly preferred are the titanium halides,especially titanium tetrachloride.

The alkoxy group of the phthalic acid ester used comprises at least fivecarbon atoms, preferably at least eight carbon atoms. Thus, as the estermay be used e.g. propylhexyl phthalate, dioctyl phthalate, di-isodecylphthalate and ditridecyl phthalate. The molar ratio of phthalic acidester and magnesium halide is preferably about 0.2:1.

The transesterification can be carried out, e.g. by selecting a phthalicacid ester—a lower alcohol pair, which spontaneously or by the aid of acatalyst, which does not damage the procatalyst composition,transesterifies the catalyst at an elevated temperature. It is preferredto carry out the transesterification at a temperature which is 110-115°C., preferably 120-140° C.

In a preferred embodiment, the Ziegler-Natta catalyst system can bemodified by polymerizing in the presence of the catalyst a vinylcompound of the formula

wherein R₁ and R₂ together form a 5 or 6-membered saturated, unsaturatedor aromatic ring or independently represent an alkyl group comprising 1to 4 carbon atoms, and the modified catalyst is used for the preparationof the polymer composition. Preferably, the vinyl compound is vinylcyclohexane. Further details about this modification are provided in EP1 028 985. The polymerized vinyl compound is acting as a nucleatingagent for the polymer composition of the present invention and furthersupports reaching high stiffness but still good impact behaviour, inparticular at low temperature.

Other nucleating agents that can be added to the polymer comprise talc,dibenzylidene sorbitol, sodium benzoate, di(alkylbenzylidene)sorbitol,or mixtures thereof. Within the context of the present invention, it isalso possible to combine one of these nucleating agents with the vinylnucleating system above. In a preferred embodiment, the polymerizedvinyl compound is used in combination with talc.

The catalyst prepared by the method described above, either modifiedwith the vinyl compound or not, is used together with an organometalliccocatalyst and with an external donor. Generally, the external donor hasthe formula

R_(n)R′_(m)Si(R″O)_(4-n-m)

whereinR and R′ can be the same or different and represent a linear, branchedor cyclic aliphatic, or aromatic group;R″ is methyl or ethyl;n is an integer of 0 to 3;m is an integer of 0 to 3; andn+m is 1 to 3.

In particular, the external donor is selected from the group consistingof cyclohexyl methylmethoxy silane (CHMMS), dicyclopentyl dimethoxysilane (DCPDMS), diisopropyl dimethoxy silane, di-isobutyl dimethoxysilane, and di-t-butyl dimethoxy silane.

An organoaluminium compound is used as a cocatalyst. The organoaluminiumcompound is preferably selected from the group consisting of trialkylaluminium, dialkyl aluminium chloride and alkyl aluminiumsesquichloride.

According to the invention, such catalysts are typically introduced intothe first reactor only. The components of the catalyst can be fed intothe reactor separately or simultaneously or the components of thecatalyst system can be precontacted prior to the reactor.

Such precontacting can also include a catalyst prepolymerization priorto feeding into the polymerization reactor proper. In theprepolymerization, the catalyst components are contacted for a shortperiod with a monomer before feeding to the reactor.

As discussed above, in preferred embodiments the homopolymer and therandom copolymer are prepared first, whereas the elastomeric copolymeris prepared in a final step. Preferably, the elastomeric copolymer isprepared in a gas phase reactor. Optionally, two or more gas phasereactors can be used. The one or more gas phase reactors for thepreparation of the elastomer can be in serial configuration with thereactors used for the preparation of the homopolymer and randomcopolymer. When using such a reactor configuration, the elastomericcopolymer is produced in the presence of the homopolymer/randomcopolymer matrix and dispersed therein.

As an alternative, the elastomeric copolymer can be prepared separatelyand mixed with the homopolymer/random copolymer matrix at a later stage,e.g. by mechanical blending.

In general, the conditions for the preparation of the elastomericcopolymer are within the limits of conventional conditions forethylene-propylene rubber (EPM) production. Typical conditions aredisclosed e.g. in Encyclopedia of Polymer Science and Engineering,second edition, vol. 6, p. 545-558. An elastomeric product is obtainedwhen the comonomer content of the polymer is within a certain range.

The catalytic system described above for the preparation of thepolypropylene homopolymer and random copolymer can also be used for thepreparation of the elastomeric copolymer.

To further improve the balance between stiffness, durability andprocessability of the polymer composition, a specific split betweenprocess steps (i) and (ii) on the one hand and process step (iii) on theother hand can be chosen. Preferably, the split between the processsteps (i) and (ii) and the process step (iii) is from 95:5 to 60:40,more preferably from 90:10 to 75:25 and even more preferably from 90:10to 80:20.

The present invention also provides a pipe and pipe fittings preparedfrom the polymer composition discussed above, e.g. by extrusion orinjection moulding. The polymer composition can be used for pressure aswell as non-pressure pipes. Preferably, it is used for non-pressurepipes. These pipes have high durability as indicated by high resistanceto slow crack growth. Furthermore, they can be prepared at high linespeed due to increased melt flow rate. Furthermore, the pipes show highstiffness.

The invention is now described in further detail by making reference toexamples.

EXAMPLES 1. Measuring Methods (a) Melt Flow Rate

Melt flow rate was measured according to ISO 1133, either at 230° C. and2.16 kg (MFR_(2.16 kg/230° C.)) or at 230° C. and 10 kg(MFR_(2.16 kg/230° C.)).

(b) Tensile Properties

Tensile properties were determined on samples prepared fromcompression-moulded plaques having a sample thickness of 4 mm.

Tensile modulus was determined according to ISO 527-2/1 B at 1 mm/min.and 23° C.

To determine stress at yield and strain at yield, a speed of 50 mm/min.was used.

(c) Resistance to Slow Crack Growth

Resistance to slow crack growth was determined according to ISO 1167. Inthis test, a specimen is exposed to constant circumferential (hoop)stress of 4.2 MPa at elevated temperature of 80° C. in water-in-water.The time in hours to failure is recorded.

The tests were performed on pipes produced on a conventional pipeextrusion equipment, the pipes having a diameter of 110 mm and a wallthickness of 5 mm.

(d) Notched Impact Strength

Notched impact strength was determined according to ISO 179/1eA onsamples prepared from compression-moulded plaques.

(e) Amount of Comonomer Units

The amount of comonomer units was determined by FTIR, calibrated by NMR.

(f) Polydispersion Index PI

The polydispersion index PI is calculated according to the followingequation:

PI=10⁵ Pa/G _(C)

wherein G_(C) in Pa is the cross over modulus at which G′=G″=G_(C).

The rheology measurements have been made according to ISO 6421-10.Measurements were made at 220° C. and 200° C. G′ and G″ indicate storagemodulus and loss modulus, respectively. Measurements were made on aPhysica MCR 300 rheometer with a plate-plate fixture, plate diameter 25mm, and a distance between the plates of 1.8 mm.

(g) SIST Measurements

Stepwise Isothermal Segregation Technique (SIST) fractionates thematerial according to the chain regularity (the average length of theisotactic PP sequences between the defects).

SIST analysis was performed on a Mettler DSC 821 e with aluminiumcrucible having a volume of 40 μm.

Sample weight 4-6 mgNitrogen flow 80 ml/minSIST analysis was done with the following temperature program:

heating/cooling isothermal/ step rate annealing 1^(st) melting  25-225°C. 10° C./min  10 min at 225° C. Crystallisation 225-155° C. 10° C./min120 min at 155° C. step, annealing 155-145° C. 10° C./min 120 min at145° C. 145-135° C. 10° C./min 120 min at 135° C. 135-125° C. 10° C./min120 min at 125° C. 125-115° C. 10° C./min 120 min at 115° C. 115-105° C.10° C./min 120 min at 105° C.  105-20° C. 10° C./min   5 min at 20° C.2^(nd) melting  20-200° C. 10° C./min —

The 2^(nd) melting curve can be used for calculation of the lamellathickness distribution according to Thomson-Gibbs equation:

$T_{m} = {T_{0}\left( {1 - \frac{2\; \sigma}{\Delta \; {H_{0} \cdot L}}} \right)}$

where T₀=457K, ΔH₀=184×10⁶ J/m³, σ=0.049.6 J/m² and L is the lamellathickness.

2. Materials

In Examples 1 to 11, polymer compositions according to the presentinvention were prepared. In Examples 1, 2, 4, 5, 7, 8 and 11, thepolypropylene random copolymer was prepared first in a loop reactor,followed by preparation of the polypropylene homopolymer in a first gasphase reactor and preparation of the elastomeric copolymer in a secondgas phase reactor. Ethylene was used as a comonomer for the randomcopolymer and the elastomer.

In Examples 3, 6, 9 and 10, the homopolymer was prepared first in a loopreactor, followed by preparation of the polypropylene random copolymerin a first gas phase reactor and preparation of the elastomericcopolymer in a second gas phase reactor. Ethylene was used as acomonomer for the random copolymer and the elastomer.

In the inventive examples 1-11, a Ziegler-Natta type catalyst was usedwhich had been modified by transesterification and polymerization of avinyl compound as described above.

Reference materials 1 to 7 are conventional impact-modified propylenepolymers, comprising a polypropylene homopolymer matrix and anelastomeric copolymer dispersed therein. In particular, referencematerials 1, 2, 3, 6 and 7 were prepared in a loop reactor, followed bya gas phase reactor and using a Ziegler-Natta catalyst including anelectron donor. Reference materials 4 and 5 were produced in two loopreactors and one gas phase reactor. For reference materials 3, 5 and 6,a Ziegler-Natta type catalyst was used which had been modified bytransesterification and polymerization of a vinyl compound as describedabove. Reference materials 1, 2, 3, 5, 6 and 7 comprise talc (less than1 wt %) as a nucleating agent.

Reference materials 8 to 11 are impact-modified propylene polymers.These polymers have a matrix comprising a polypropylene homopolymer or,as an alternative, a polypropylene having less than 1 wt % ethyleneunits in combination with a polypropylene random copolymer. Referencematerials 8 to 11 were prepared according to examples 10 to 13 of EP 0894 103.

In Table 1, the reaction conditions for Examples 1 to 11 are summarized.

TABLE 1 Summary of reaction conditions Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex.6 Ex. 7 Ex. 8 Ex. 9 Ex. 10 Ex. 11 Al/Ti ratio (mol/mol) 200 200 200 200200 200 200 200 200 200 200 Al/donor ratio (mol/mol) 5.0 5.0 5.0 5.0 5.05.0 5.0 5.0 5.2 5.2 5.2 Loop: Temperature (° C.) 80.0 80.0 80.0 80.070.0 80.0 80.0 80.0 80.0 80.0 80.0 Split % 54 53 48 54 50 49 56 56 52 5240 MFR2 (g/10 min) 0.04 0.06 0.42 0.10 0.06 0.35 0.10 0.20 0.73 0.420.22 XS/(%) 3.6 3.4 1.9 3.4 6.4 1.5 3.4 3.4 1.8 1.8 5.1 C2 content (%)2.1 1.9 2.3 4.7 2.2 2.2 2.8 GPR1: Temperature (° C.) 95 95 85 95 95 8595 95 85 85 95 Split % 46 47 52 46 50 51 44 44 48 48 60 MFR2 (g/10 min)0.2 0.3 0.3 0.5 0.3 0.3 0.5 0.9 0.7 0.3 3.4 XS (%) 2.2 2.1 2.1 2.2 3.43.3 2.2 2.2 2.2 2.2 3.6 Ethene content (%) 1.0 0.9 1.1 1.1 2.4 2.4 1.21.2 1.0 1.0 1.3 GPR2: Temperature (° C.) 70 60 60 60 60 60 60 60 60 6060 C2/C3 ratio (mol/kmol) 434 580 643 617 546 653 608 608 645 645 546MFR2 (g/10 min) 0.24 0.32 0.26 0.46 0.27 0.23 0.48 0.75 0.63 0.31 3.3 XS(%) 11.3 8.1 12.6 10.6 10.1 13.1 10.7 10.7 10.8 10.8 10.5 ethene of AM32 35 34 36 41 32 36 35 37 37 41 viscosity of AM (dl/g) 3.2 3.5 3.3 3.43.4 2.9 3.3 3.3 3 3 2.4 Ethene content (%) 5.2 4.2 6 5.4 5.5 6.3 5.4 5.45 5 5.6 Pellet MFR2 (g/10 min) 0.26 0.35 0.32 0.47 0.27 0.28 0.44 0.750.63 0.3 3.2

In Tables 2-4, melt flow rate, tensile modulus, impact strength andresistance to slow crack growth are summarized for the inventiveexamples and the reference materials.

When compared to reference materials 1-7, the inventive materials havecomparable or even better flowability (i.e. higher melt flow rate) andcomparable stiffness (indicated by tensile modulus values) but have aresistance to slow crack growth which is several times higher. Thus,durability of the inventive compositions is very much improved.

When having a specific look at reference material 4, the conflictingbehaviour of melt flow rate and durability is clearly evident. However,in inventive example 4 and in particular in inventive examples 8 and 11,the melt flow rate was increased but still results in materials ofimproved durability.

When compared to reference materials 8 to 11, the inventive materialshave significantly higher tensile modulus and melt flow rate, therebyimproving stiffness and processability.

From the SIST data, in particular when comparing Ex. 8 and Ref. 4, itcan be seen that the present invention provides the advantage offlexibility with a low amount of the fraction with big lamellaes >17.6nm (big lamellaes mean high stiffness) but still results in highstiffness.

TABLE 2 Homo-Random-Elastomer example materials Ex. 3 Ex. 6 Ex. 9 Ex. 10MFR (1) g/10 min 0.31 0.3 0.63 0.3 Melting peak ° C. 162.3 161.8 162.5163.7 T_(m) (2) Crystallization ° C. 127.3 127.6 127.7 126.2 temp. (2)Tensile MPa 1478 1271 1440 1562 modulus (3) Stress at yield MPa 28.426.2 28.3 28.8 (3) Strain at yield % 6.8 8.4 6 6.5 (3) NIS, 23° C. KJ/m²64 60.3 61.5 61.3 (4) NIS, −20° C. KJ/m² 3.8 3.6 3.8 3.7 (4) SCG, 80°C./4.2 h 1407 4824 R 624 R 5880 R MPa (5) 1685 624 R Polydisp. index 3.13.1 2.8 3.3 PI (6) (1) ISO 1133, condition 230° C., 2.16 kg (2) DSC,using a temperature increasing/decreasing rate of 10° C./min. (3) ISO527, speed 50 mm/min for stress at yield and strain at yield. Fortensile modulus speed 1 mm/min. (4) ISO 179/1eA; (5) ISO 1167; (R =still running) (6) Rheometer plate/plate, 220° C.

Mechanical tests from 4 mm compression moulded plaques, which were 3weeks±2 days old when tested.

For additivation, a conventional system is used containing a lubricant,antioxidant and process stabilizer.

TABLE 3 Random-Homo-Elastomer example material Ex. 1 Ex. 2 Ex. 4 Ex. 5Ex. 7 Ex. 8 Ex. 11 MFR g/10 min 0.24 0.31 0.46 0.32 0.44 0.75 3.3 (1)Melting peak ° C. 159.5 159.5 158.8 155.6 159.1 158.9 158.1 T_(m) (2)Crystallization ° C. 124.8 125.4 125.9 122.8 125.7 125.7 125.9 temp. (2)Tensile MPa 1390 1563 1596 1233 1435 1465 1413 modulus (3) Stress atyield MPa 27.1 30.8 29.3 27.3 28.4 27.5 27.3 (3) Strain at yield % 6.76.4 6.6 8.4 6.6 6.1 5 (3) NIS, 23° C. kJ/m² 62.7 20.1 18.1 51.8 48.317.7 13.4 (4) NIS, −20° C. kJ/m² 5.4 3 3.7 2.1 3.9 4.9 4.7 (4) SCG, 80°C./4.2 h 5544 R 5544 R 5328 R 5544 R 4392 R 648 R 912 R MPa (5) 6600 R6600 R 6600 R 4632 R 563 603 Polydisp. index 4.4 4.5 3.8 4.5 3.9 3.3 (6)SIST/melt % 5.9 fraction of lamellar >17.6 nm (7) (1)-(6) have the samemeaning as in Table 2. (7) SIST, Stepwise Isothermal SegregationTechnique

Mechanical tests from 4 mm compression moulded plaques, which were 3weeks±2 days old when tested.

For additivation, a conventional system is used containing a lubricant,antioxidant and process stabilizer.

TABLE 4 Reference materials - Properties Ref. 1 Ref. 2 Ref. 3 Ref. 4Ref. 5 Ref. 6 Ref. 7 Ref. 8 Ref. 9 Ref. 10 Ref. 11 MFR g/10min 0.28 0.280.3 0.8 0.27 0.35 1.3 0.13 0.14 0.19 0.16 (1) Melt. peak ° C. 166 166.1166.4 164.1 167 166 164.7 154.7 154.6 150.9 158 T_(m) (2) Cryst. ° C.120.9 117.7 129.5 temp. (2) Tensile MPa 1409 1429 1528 1360 1826 16531495 940 860 860 880 modulus (3) Stress at yield MPa 25.5 25.6 25.2 24.928.5 24.2 24.8 23.3 21.3 21.7 20.5 (3) Strain at yield % 5.5 5.4 5.2 5.24.1 4.4 3.3 9.8 9.2 10.5 7.5 (3) NIS, 23° C. kJ/m² 63.2 65.5 68.6 54.231.5 48.2 48.3 53 69 66 56 (4) NIS, −20° C. kJ/m² 6.1 6.3 5.9 4.7 4.26.4 5.7 2.3 3.7 3.4 5.2 (4) SCG, 80° C./4.2 h 132, 145, — 159, 215 67,95 499, 667 96, 103, 74 MPa (5) 386 111 Polydisp. index 3.6 3.6 3.7 3.85.8 3.5 3.6 6.1 7.4 (6) SIST/melt % 48.9 3.2 39.7 fraction oflamellar >17.6 nm (7) (1)-(7) as defined above.

Mechanical tests from 4 mm compression moulded plaques, which were 3weeks±2 days old when tested.

PI of Ref. 7, 9 and 10 was measured at a temperature of 200° C.

1. A polymer composition, comprising (i) a polypropylene homopolymer,(ii) a polypropylene random copolymer, prepared by copolymerization ofpropylene with an olefin comonomer and having an amount of olefincomonomer units of 0.2 to 5.0 wt %, and (iii) an elastomeric copolymerof propylene and at least one olefin comonomer, the polymer compositionhaving a tensile modulus, determined according to ISO 527-2/1 B at 1mm/min. and 23° C., of at least 1200 MPa.
 2. The polymer compositionaccording to claim 1, wherein the polypropylene homopolymer fraction hasa melt flow rate MFR_(2.16 kg/230° C.) of less than 20 g/10 min.
 3. Thepolymer composition according to claim 1, wherein the weight averagemolecular weight of the polypropylene random copolymer is higher thanthe weight average molecular weight of the polypropylene homopolymer. 4.The polymer composition according to claim 1, wherein the olefincomonomer of the polypropylene random copolymer is selected fromethylene, C₄ to C₁₀ alpha-olefins, or mixtures thereof.
 5. The polymercomposition according to claim 4, wherein the olefin comonomer isethylene.
 6. The polymer composition according to claim 5, wherein thepolypropylene random copolymer comprises an amount of 0.2 to 3.0 wt % ofethylene comonomer units.
 7. The polymer composition according to claim1, wherein the amount of olefin comonomer units within the polypropylenerandom copolymer is from 0.1 wt % to 3.0 wt %, based on the combinedweight of the polypropylene homopolymer and the polypropylene randomcopolymer fraction.
 8. The polymer composition according to claim 1,wherein the polypropylene homopolymer and the polypropylene randomcopolymer are blended during polymerization to result in apolypropylene-based matrix.
 9. The polymer composition according toclaim 8, wherein the polypropylene-based matrix has a melt flow rateMFR_(2.16 kg/230° C.) within the range of 0.1 g/10 min. to 10.0 g/10min.
 10. The polymer composition according to claim 1, wherein theolefin comonomer of the elastomeric copolymer is selected from ethylene,C₄ to C₁₀ alpha-olefins, or mixtures thereof.
 11. The polymercomposition according to claim 10, wherein the olefin comonomer isethylene and the elastomeric copolymer comprises an amount of 10 to 70wt % of ethylene comonomer units.
 12. The polymer composition accordingto claim 1, having a melt flow rate MFR_(2.16 kg/230° C.) and aresistance to slow crack growth which satisfy the followingrelationship:t≧300−200*MFR_(2.16 kg/230° C.) for 0.1 g/10min≦MFR_(2.16 kg/230° C.)<1.0 g/10 min,t≧105−5*MFR_(2.16 kg/230° C.) for 1.0 g/10min≦MFR_(2.16 kg/230° C.)<10.0 g/10 min,t≧55 for MFR_(2.16 kg/230° C.)≧10.0 g/10 min, wherein t in hours is thetime of failure in the slow crack growth test performed at 80° C. and4.2 MPa according to ISO
 1167. 13. The polymer composition according toclaim 1, having a melt flow rate MFR_(2.16 kg/230° C.) of at least 0.2g/10 min.
 14. The polymer composition according to claim 13, having amelt flow rate MFR_(2.16 kg/230° C.) of at least 0.2 g/10 min but lessthan 10.0 g/10 min.
 15. The polymer composition according to claim 1,comprising the elastomeric copolymer in an amount of 5 to 15 wt %. 16.The polymer composition according to claim 1, having a Charpy impactstrength of at least 2.0 kJ/m², measured according to ISO 179/1 eA at−20° C.
 17. The polymer composition according to claim 16, wherein thetensile modulus is at least 1375 MPa and the Charpy impact strength at−20° C. is at least 3.0 kJ/m².
 18. The polymer composition according toclaim 1, further comprising a nucleating agent selected from talc,polymerized vinyl compounds, dibenzylidene sorbitol, sodium benzoate,di(alkylbenzylidene)sorbitol, or mixtures thereof.
 19. The polymercomposition according to claim 18, wherein the nucleating agent is apolymerized vinyl compound.
 20. A process for preparing the polymercomposition according to claim 1, comprising the following steps in anysequence: (i) preparing a polypropylene homopolymer, (ii)copolymerization of propylene with an olefin comonomer to result in apolypropylene random copolymer, and (iii) copolymerization of propylenewith an olefin comonomer to result in an elastomeric copolymer.
 21. Theprocess according to claim 20, wherein the process steps (i) and (ii)are carried out in at least one loop reactor and/or at least one gasphase reactor.
 22. The process according to claim 20, wherein allprocess steps (i) to (iii) are carried out in at least one loop reactorand/or at least one gas phase reactor.
 23. The process according toclaim 20, wherein the process steps are carried out in the followingsequence: (i)→(ii)→(iii).
 24. The process according to claim 20, whereinthe process steps are carried out in the following sequence:(ii)→(i)→(iii).
 25. The process according to claim 21, wherein the firstprocess step is carried out in a loop reactor, optionally followed bypolymerization in a gas phase reactor, and the second and third processstep are carried out in separate gas phase reactors.
 26. The processaccording to claim 21, wherein the one or more loop reactors areoperated at a temperature of at least 70° C. and a pressure of 4600 to10000 kPa.
 27. The process according to claim 26, wherein at least oneloop reactor is operated at supercritical conditions.
 28. The processaccording to claim 21, wherein the one or more gas phase reactors forpreparing the polypropylene homopolymer and/or polypropylene randomcopolymer are operated at a temperature of 60° C. to 100° C. and apressure of 1000 kPa to 4000 kPa.
 29. The process according to claim 28,wherein the split between the process step (i) and the process step(ii), irrespective of their sequence, is from 80:20 to 20:80.
 30. Theprocess according to claim 20, using a Ziegler-Natta catalyst includingan electron-donor.
 31. The process according to claim 30, wherein theZiegler-Natta catalyst comprises a cocatalyst component which has beenprepared by bringing together magnesium dichloride, a lower alcoholselected from methanol, ethanol or mixtures thereof, a titanium compoundand an ester of phthalic acid having an alkoxy group of at least fivecarbon atoms.
 32. The process according to claim 31, wherein theZiegler-Natta catalyst is modified by polymerizing, in the presence ofthe catalyst, a vinyl compound of the formula

wherein R₁ and R₂ together form a 5 or 6-membered saturated, unsaturatedor aromatic ring or independently represent an alkyl group comprising 1to 4 carbon atoms, and the modified catalyst is used for the preparationof the polymer composition.
 33. The process according to claim 20,wherein a nucleating agent is added selected from talc, polymerizedvinyl compounds, dibenzylidene sorbitol, sodium benzoate,di(alkylbenzylidene) sorbitol, or mixtures thereof.